Processes controlling the isotopic composition of steam and water discharges from steam vents and steam-heated pools in geothermal areas

On the basis of isotopic analyses of steam and water discharges from the Wairakei, El Tatio and The Geysers geothermal areas, underground steam separation from the rising geothermal fluid appears to be adequately described in terms of a single-step process at temperatures of around 230°C. Absorption of this steam into nearly stagnant pools gives rise to the formation of isotopically enriched waters with compositions following a line with slope σ = ε′_D / (Δ_rw + ε′_¹⁸O − ε_{¹⁸O .230°C}), where ε′_D and ε′_¹⁸O are the effective kinetic isotope fractionation factors (50‰ and 16‰) for steam heated pools, ε_{¹⁸O .230°C} is the equilibrium fractionation factor for oxygen-18 at 230°C (2‰) and δ_rw is the difference in ¹⁸O-content of deep chloride and local groundwater (oxygen shift) respectively. The sulfate content of these pools is a function of the proportion of steam absorbed and its H₂S-content.     

3‐E analysis of advanced power plants based on high ash coal

The objective of the study is to identify the ‘best’ possible power plant configuration based on 3‐E (namely energy, exergy, and environmental) analysis of coal‐based thermal power plants involving conventional (subcritical (SubC)) and advanced steam parameters (supercritical (SupC) and ultrasupercritical (USC)) in Indian climatic conditions using high ash (HA) coal. The analysis is made for unit configurations of three power plants, specifically, an operating SubC steam power plant, a SupC steam power plant, and the AD700 (advanced 700°C) power plant involving USC steam conditions. In particular, the effect of HA Indian coal and low ash (LA) reference coal on the performance of these power plants is studied. The environmental impact of the power plants is estimated in terms of specific emissions of CO₂, SO_x, NO_x, and particulates. From the study, it is concluded that the maximum possible plant energy efficiency under the Indian climatic conditions using HA Indian coal is about 42.3% with USC steam conditions. The results disclose that the major energy loss is associated with the heat rejection in the cooling water, whereas the maximum exergy destruction takes place in the combustor. Further, the sliding pressure control technique of load following results in higher plant energy and exergy efficiencies compared to throttle control in part‐load operation. Copyright © 2009 John Wiley & Sons, Ltd.     

Effects of water vapor addition on the laminar burning velocity of oxygen-enriched methane flames

The effects of steam addition on the laminar burning velocity of premixed oxygen-enriched methane flames are investigated at atmospheric pressure. Experiments are carried out with an axisymmetric burner on which laminar conical flames are stabilized. A newly devised steam production system is used to dilute the reactants with water vapor. The oxygen-enrichment ratio in the oxidizer, defined as O₂/(O₂ + N₂) (mol.), is varied from 0.21 (air) to 1.0 (pure oxygen). The equivalence ratio ranges from 0.5 to 1.5 and the steam molar fraction in the reactive mixture is varied from 0 to 0.50. For all compositions examined, the reactive mixture is preheated to a temperature T_u = 373 K. Laminar flame speeds are determined with the flame area method using a Schlieren apparatus. The deviations induced by stretch effects due to aerodynamic strain and flame curvature are assessed using Particle Imaging Velocimetry measurements and flame images, and these data are used to estimate the uncertainty of the flame speed measurements. The experiments are completed by numerical simulations conducted with the PREMIX code using different detailed kinetic mechanisms. It is shown that the laminar flame speed of CH₄/O₂/N₂/H₂O_(v) mixtures features a quasi-linear decrease with increasing steam molar fraction, even at high steam dilution rates. Numerical predictions are in good agreement with experimental data for all compositions explored, except for low dilution rates X_H2O<0.10 in methane–oxygen mixtures, where the flame speed is slightly underestimated by the calculations. It is also shown that steam addition has a non-negligible chemical impact on the flame speed for methane–air flames, mainly due to water vapor high chaperon efficiency in third-body reactions. This effect is however strongly attenuated when the oxygen concentration is increased in the reactive mixture. For highly oxygen-enriched flames, steam can be considered as an inert diluent.     

Optimizing the preparation of Ni-Ce-Pr catalysts for efficient hydrogen production by n-dodecane steam reforming

Nowadays, the development of fuel cell is getting more and more inseparable from the production of hydrogen. Long-chain hydrocarbons steam reforming is a feasible way for hydrogen supply. Herein, various nickel-ceria-praseodymium (Ni-Ce-Pr) catalysts have been prepared by a sol-gel method. Multiple parameters during catalyst preparation, including the amount of Ni, the content of doped Pr and the calcination temperature, were systematically studied for tuning the catalytic performance for n-dodecane steam reforming in a fixed-bed reactor under 15 mL/g_cat·h at 600°C and water-to-carbon molar ratio of 2 at 0.1 MPa. Reaction data showed that both the amount of Ni and the content of doped Pr will greatly affect the n-dodecane conversion and hydrogen production. Additionally, the calcination temperature during catalyst preparation showed a big influence on its performance for n-dodecane steam reforming. After optimization, 10Ni-CePr₂-600 exhibits the highest activity and stability for n-dodecane steam reforming, accompanying with the lowest rate of coke deposition (0.015 g_c/g_cat·h). The structure and oxygen vacancy of the catalyst was characterized by H₂-TPR, Raman, and X-ray photoelectron spectroscopy (XPS). The superior activity and stability of 10Ni-CePr₂-600 are ascribed to the strong interaction between NiO and support along with abundant oxygen vacancies in the Pr-doped ceria.     

One feedwater heater taken out of service as a strategy to maintain full load and its effect on steam power cycle parameters and performance

The aim of this study is to analyze the suitability of one heater removal as a strategy for maintaining full load operation of steam power cycles when superheated and/or reheated temperatures (T_SS, T_RS) decrease and the effect on the net heat rate (HR_Net). For this purpose, three regenerative cycles with different numbers of closed feedwater heaters were chosen. The cycles were analyzed at different steady states with Thermoflex software. Removing a heater has an important influence on the cycle operation and performance, leading to the redistribution of extraction mass flows, with the heater immediately downstream being the most affected. This may make it necessary to reduce the load of the cycle. However, when the highest pressure heater (highest PH) is removed from service, the changes are not so significant. When T_SS and/or T_RS decrease, the plant may not achieve full load operation. Nevertheless, if the highest PH is removed from service, it can help to recover full load. This is due to the decrease in the water/steam mass flow through the steam generator, which produces an increase in T_SS and/or T_RS. On the one hand, this measure leads to higher HR_Net in comparison to that of the nominal conditions. On the other hand, there are certain conditions at which HR_Net is lower than when all the heaters are in service and the values of T_SS and/or T_RS are low. Thus, for maintaining full load, the highest PH removal can be applied and cycle parameters optimized in order to reach a HR_Net closer to its nominal value. The higher the number of closed feedwater heaters, the more adequate is the application of this strategy.     

Hot spring activity in the Dakongbeng geothermal area, China

The Dakongbeng geothermal area, whose hot springs reach a temperature of up to 96°C, has been considered one of the potential high-temperature hydrothermal systems in south-west China. The concentration of dominant cations and anions indicates an NaHCO₃ type of thermal water, whose major constituents in decreasing order are: Na>K>Ca>Mg, HCO₃>SiO₂>Cl>SO₄. On the basis of the silica geothermometer, cation geothermometers, gas geothermometer and activity diagram, the reservoir temperature is estimated at about 200°C. All the thermal waters have originated from meteoric water of a higher altitude that circulated as ground water at considerable depth along faults. The stability of their contents of Cl, SiO₂, δD, δ¹⁸O and of the Cl/B, Na/Li ratios suggests that the main heat loss process is through steam loss. The geochemistry of the initial liquid has been estimated by single and continuous steam loss. On the basis of its geologic and geographic setting, the Dakongbeng geothermal area appears to belong to the Himalayan geothermal belt and is thus regarded as an area of interest for further study.     

Thermodynamic analysis of steam gasification of municipal solid waste

The main objective of this work is to understand steam gasification performance of municipal solid wastes (MSWs). The thermodynamic analysis was carried out by using seven different kinds of MSWs as the feedstock with the temperature ranging from 773 to 1773 K and steam to MSW ratio ranging from 0.5 to 1.5. It is summarized that temperature and the effective mole ratio of H/C (R_H/C) are two key factors, and lower temperature and/or larger R_H/C result in a larger mole ratio of H₂/CO. The model predictions agree very well with the referenced experimental results, which can predict steam gasification performance of MSWs.     

Typical off-design analytical performances of internal combustion engine cogeneration

Based on experimental data, typical off-design characteristic curves with corresponding formulas of internal combustion engine (ICE) are summarized and investigated. In combination with analytical solution of single-pressure heat recovery steam generator (HRSG) and influence of ambient pressure on combined heat and power (CHP) system, off-design operation regularities of ICE cogeneration are analyzed. The approach temperature difference ΔT _a, relative steam production and superheated steam temperature decrease with the decrease in engine load. The total energy efficiency, equivalent exergy efficiency and economic exergy efficiency first increase and then decrease. Therefore, there exists an optimum value, corresponding to ICE best efficiency operating condition. It is worth emphasizing that ΔT _a is likely to be negative in low load condition with high design steam parameter and low ICE design exhaust gas temperature. Compared with single shaft gas turbine cogeneration, ΔT _a in ICE cogeneration is more likely to be negative. The main reason for this is that the gas turbine has an increased exhaust gas flow with the decrease in load; while ICE is on the contrary. Moreover, ICE power output and efficiency decrease with the decrease in ambient pressure. Hence, approach temperature difference, relative steam production and superheated steam temperature decrease rapidly while the cogeneration efficiencies decrease slightly. It is necessary to consider the influence of ambient conditions, especially the optimization of ICE performances at different places, on cogeneration performances.     

Optimal design of gas turbine CHP plant with preheater and HRSG

Located in the south of Iran, Jiroft Paper Mill Company requires an integrated combined heat and power plant, which can provide 50 MW of electric power and 100 ton h^?1 saturated steam at 13 bar, to produce paper from an adjacent eucalyptus forest. The plant is composed of an air compressor, combustion chamber, air preheater, turbine, as well as a heat recovery steam generator. The design parameters of the plant were chosen as: compressor pressure ratio (r_c), compressor isentropic efficiency (η_AC), gas turbine isentropic efficiency (η_T), combustion chamber inlet temperature (T₃), and turbine inlet temperature (T₄). In order to optimally find the design parameters a thermoeconomic approach has been followed. An objective function representing the total cost of the plant in terms of dollar per second was defined as the sum of the operating cost related to the fuel consumption and the capital investment for equipment purchase and maintenance costs. Subsequently, different parts of the objective function have been expressed in terms of decision variables. Finally, the optimal values of decision variables were obtained by minimizing the objective function using sequential quadratic programming. The influence of changes in the demanded power and steam on the design parameters has also been studied for 40, 50, 60, and 70 MW of net power output, and 100, 120, and 150 ton h^?1 of saturated steam mass flow rate. Finally, the sensitivity analysis of change in design parameters with change in fuel or investment cost was performed. Copyright © 2009 John Wiley & Sons, Ltd.     

Hydrogen in steel exposed to geothermal fluids

The concentration of hydrogen in carbon steels exposed to geothermal fluids at Broadlands (New Zealand) was determined by simultaneous exposures of hydrogen and Corrosometer probes. Iron dissolution and hydrogen permeation in thin-walled cylinders satisfied a simple overall reaction (1 + x) Fe ± H₂S → Fe_{(1 ± x)}S + H₂ based on scale compositions determined from coupons. The influence of time and temperature over the range 20–160°C on corrosion rates of carbon steels was studied. Hydrogen concentrations were calculated, making allowance for traps, as functions of radial position and cylinder wall thickness. Application of these results to the design of a full-size steam turbine rotor is discussed.     

宝钢中压蒸汽管网疏水系统的节能研究

工业企业内的节能工作已经成为企业可持续发展的主题,作为能源消耗中的蒸汽系统,目前在宝钢内部的损失率居高不下。介绍了宝钢中压蒸汽疏水器的现状,通过试验对节能型疏水器与普通疏水阀的性能进行了对比,并对节能型疏水阀的经济性进行分析,并提出了宝钢中压蒸汽疏水系统改造方案。     

Steam exfoliation of graphitic carbon nitride as efficient route toward metal-free electrode materials for hydrogen production

This study demonstrates the potential of obtaining nanostructured materials based on g-C₃N₄ with a high specific surface area for use as efficient electrode materials for hydrogen production. The study uses a novel method of g-C₃N₄ exfoliation that increases the specific surface area of the starting material by a factor of three. Nanocrystalline g-C₃N₄ is obtained through the thermolysis of urea and treated with steam in a specified temperature range. The resulting series is analyzed using a range of physicochemical methods to determine the optimal temperature for steam exfoliation. Catalytic electrochemical tests are carried out in the electrolytic reforming of ethanol. It has been demonstrated that steam exfoliation can boost the rate of electrocatalytic reforming by 1.3 times while decreasing the amount of hydrogen evolution overpotential. The results of this study demonstrate the potential for the use of steam exfoliation as an effective method for obtaining high-performing electrode materials for hydrogen production.     

Charge transfer properties of BaCe0.88Nd0.12O3−δ co-ionic electrolyte

Charge and mass transfer in an electrochemical cell, based on an electrolyte possessing both oxygen ion and proton conductivity (co-ionic electrolyte), under the gradient of steam partial pressure was considered from theoretical point of view. It is shown that, due to simultaneous carry of oxygen ions and protons in the same directions, steam permeation through the co-ionic electrolyte occurs. Experiments performed with the BaCe_0.88Nd_0.12O_3−δ have confirmed the existence of steam permeation through the co-ionic electrolyte.     

Adiabatic fixed-bed gasification of coal, dairy biomass, and feedlot biomass using an air–steam mixture as an oxidizing agent

Concentrated animal feeding operations, such as cattle feedlots and dairies, produce a large amount of manure, cattle biomass (CB), which can be included as renewable feedstock for locally based gasification for syngas (CO and H₂) production and subsequent use in power generation. Experimental results on effects of bed temperature and gas composition on the higher heating value (HHV) and energy recovery are presented for dairy biomass (DB) gasification using air and air–steam as oxidizers. Some experimental data are compared with adiabatic gasification modeling which includes atom balance conservation for assumed product species and chemical equilibrium analysis. Wyoming sub-bituminous coal (WYC) and Texas Lignite coal (TXL) are used as standard fuels for comparison purposes in modeling studies. Two main parameters are investigated in this study. One is the modified equivalence ratio (ER_M) defined as the ratio of stochiometric oxygen to total oxygen supplied in the oxidizing mixture of air and steam. The second is a measure of how much steam is in the oxidizer and is called the air steam ratio (ASTR), which is defined as the ratio of oxygen supplied in the air to the total oxygen supplied in the oxidizer. The results suggested that gasification of CB and coals under higher ER_M yield elevated concentrations of CO and CH₄, and low percentages of H₂ and CO₂, while higher ASTRs (less steam) produced mixtures poor in H₂, CO₂, and CH₄ and rich in CO with lower HHV. It was also found that FB and DB produced higher amounts of H₂ than WYC and TXL under the same ER_M and ASTR.     

Quantification of losses in thin-film polycrystalline solar cells

Comparison of measured solar-cell parameters with calculations for ideal cells is a powerful tool to assist fundamental understanding and to focus on the most effective fabrication procedures. The emphasis here will be on quantitative separation of individual loss mechanisms in polycrystalline thin-film cells based on CdTe, CuInSe₂ (CIS), and related alloys such as CuIn_1−xGa_xSe₂ (CIGS). Several techniques to facilitate separation of losses are described.     

Investigation of Indonesian low rank coals gasification in a fixed bed reactor with K2CO3 catalyst loading

Catalytic steam gasification is considered one of promising technologies for converting solid carbonaceous feedstocks into hydrogen-rich syngas, which is an important source of hydrogen for various industrial sectors. The K₂CO₃-catalyzed steam gasification of low rank coals (LRCs) was conducted in a fixed bed reactor for elucidating the effects of gasifying temperature and catalyst loading amount on hydrogen yield. Hydrogen-rich syngas can be obtained at gasifying temperature of 800 °C and loading amount of 10 wt% K₂CO₃. The loading amount of 10 wt% K₂CO₃ was the saturation point and provided a good gasification reactivity in catalytic steam gasification of three LRCs. The experimental data of these three LRCs were well described by the random pore model (RPM). The RPM fitted the experimental data at 800 °C better than the experimental data obtained at 700 °C and 600 °C. Reactivity index (R_0.5), activation energy (Ea) and reaction rate constant (k) were also used to predict the characteristics of the K₂CO₃-catalyzed steam gasification process. Catalytic steam gasification utilizing the mixture of three LRCs as a feedstock was also investigated and displayed X_C of 86.22% and 0.95 mol mol^?1-C, indicating a good feasibility and potential industrial applications.     

Investigation of pollutants dispersion from power stations

Theoretical investigations were conducted of pollutants dispersion, NO_x, SO₂ and Particulate Matter (PM), from stacks of arbitrary four power plants in Libya, e.g. North Benghazi, South Tripoli, Zweitina, Khoms. The first four stations are gas power plants, while the last one is gas and steam station. Gaussian plume model has been used to identify ground‐level NO_x concentrations profile downwind and crosswind of the chimneys through urban regions and also the location of maximum pollutant concentrations. The study is based on the worst‐case emission conditions of Pasquill stability categories (class D). Results indicate that maximum ground‐level NO_x impacts for all plants locate at a distance of approximately 1.0–2.5 km from stacks. The site most critical to ambient air NO_x impact is Zweitina, where the plant site is in direct vicinity to residential areas. Khoms electric station exhibits the maximum emitted NO_x, SO₂, and PM intensity, about 305, 48, and 0.7 µg/m³, respectively, that is lower than allowable concentrations recommended by World Health Organization. Copyright © 2006 John Wiley & Sons, Ltd.     

Reactivity investigation on chemical looping gasification of biomass char using nickel ferrite oxygen carrier

Chemical looping gasification (CLG) involves the use of an oxygen carrier (OC) which transfers oxygen from air to solid fuel to convert the fuel into synthesis gas, and the traditional gasifying agents such as oxygen-enriched air or high temperature steam are avoided. In order to improve the reactivity of OC with biomass char, facilitating biomass high-efficiency conversion, a compound Fe/Ni bimetallic oxide (NiFe₂O₄) was used as an OC in the present work. Effect of OC content and oxygen sources on char gasification were firstly investigated through a TG reactor. When the OC content in mixture sample attains 65 wt.%, the sample shows the maximum weight loss rate at relatively low temperature, indicating that it is very favorable for the redox reactions between OC and biomass char. The NiFe₂O₄ OC exhibits a good performance for char gasification, which is obvious higher than that of individual Fe₂O₃ OC and mechanically mixed Fe₂O₃ + NiO OC due to the Fe/Ni synergistic effect in unique spinel structure. According to the TGA experimental results, effect of the steam content and cyclic numbers on char gasification were investigated in a fixed bed reactor. Either too low steam content or too high steam content doesn't facilitate the char gasification. And suitable steam content of 56.33% is determined with maximum carbon conversion of 88.12% and synthesis gas yield of 2.58 L/g char. The reactivity of NiFe₂O₄ OC particles shows a downtrend within 20 cycles (～64 h) due to the formation of Fe₂O₃ phase, which is derived from the iron element divorced from the Fe/Ni spinel structure. Secondly, the sintering of OC particles and ash deposit on the surface are also the reasons for the deactivation of NiFe₂O₄ OC. However, the carbon conversion and synthesis gas yield at the 20th cycle are still higher than those of the blank experiment. It indicates that the reactivity of NiFe₂O₄ OC can be maintained at a relatively long time and NiFe₂O₄ material can be used as a good OC candidate for char gasification in the long time running.     

Solar steam reforming for enriched methane production: Reactor configurations modeling and comparison

A solar low-temperature steam reforming process for the production of an Enriched Methane (EM) mixture composed by CH₄ and H₂ (20%vol) exploiting the solar energy stored in a Molten Salt stream heated up by a Concentrating Solar Plant (MS-CSP) is presented and simulated through a two-dimensional steam reforming reactor model.     

Modification strategies for enhancing anti-coking of Ni-, Co-based catalysts during ethanol steam reforming: A review

Producing hydrogen from ethanol steam reforming (ESR) is a carbon-neutral and environment-friendly method, which has been expected to gradually reduce excessive emission of environmental pollution and over-exploitation of fossil resources. Low-cost nickel (Ni) and cobalt (Co) are considered the most promising active metals for industrial ESR catalysts, with the challenge that carbon deposition on such catalysts causes active site loss which limits their application. In this review, comprehensive knowledge on the ESR reaction mechanism and carbon deposition process were summarized. Based on understanding of the reaction mechanism, an anti-coking strategy keeping a balance between C–C bond scission and oxidation of hydrocarbon species was proposed. Two aspects of this strategy, including (i) enhanced C–C bond scission capability of metal, (ii) promoting effects of support for protecting the activity of metal particles and removing surface carbon, were particularly described. The revelation between the intermediate reaction and modification strategy enables the successful design of new and stable catalysts for improving anti-coking ability. This review not only shed light to the development of high-performance industrial ESR catalysts, but also contribute an innovative perspective to understand anti-coking mechanism for steam reforming of CH₃CHO, CH₃COOH, CH₃COCH₃, and even crude bio-oil.